(a) Field The present invention relates to a downlink frame generation method and a cell search method, and particularly relates to a method for generating a downlink frame and a method for searching for a cell by using the downlink frame in an orthogonal frequency division multiplexing (OFDM)-based cellular system.
(b) Description of the Related Art
In a direct sequence code division multiple access (DS-CDMA) system, the code hopping method is applied to a pilot channel in order to acquire cell synchronization and appropriate cell identification information. The code hopping method introduces a code hopping technique to the pilot channel so that a terminal may easily search for the cell without an additional synchronization channel. However, since the number of channels that are distinguishable by the frequency domain in the symbol interval is much greater than the number of channels that are distinguishable by the CDMA spread in one time domain symbol interval in the OFDM system, the usage of the time domain may waste resources with respect to capacity, and hence it is difficult to apply the code hopping method to the pilot channel time domain of the OFDM-based system. Therefore, it is desirable in the OFDM case to search for cells by efficiently using the received signals in the time domain and the frequency domain.
A conventional technique for searching for cells in the OFDM system includes divide a frame into four time blocks and allocating synchronization information and cell information. The technique proposes two frame structures. The first frame structure allocates synchronization recognition information, cell group recognition information, appropriate cell recognition information, and synchronization recognition information to four time blocks. The second frame structure allocates synchronization recognition information and appropriate cell recognition information to the first time block and the third time block, and synchronization recognition information and cell group recognition information to the second time block and the fourth time block.
In the case of following the first scheme, since symbol synchronization is acquired in the first time block, it is impossible to acquire fast synchronization within the standard of 5 ms when a terminal is turned on or in the case of a handover between heterogeneous networks. Also, it is difficult to acquire a diversity gain through accumulation of synchronization recognition information for fast synchronization acquisition.
In the case of following the second scheme, the cell search process is complicated and it is difficult to search for the cells quickly since it is required to acquire synchronization and simultaneously correlate appropriate cell recognition information or cell group recognition information so as to acquire frame synchronization.
Another method for searching for cells using an additional preamble to acquire synchronization and search for the cells has been proposed, but it is inapplicable to a system having no preamble. Further, since the preamble is disposed at the front part of the frame, the terminal must stand by for the next frame when attempting to acquire synchronization at a time position other than the first time position of the frame. Particularly, when the terminal performs a handover among the GSM mode, the WCDMA mode, and the 3GPP LTE mode, it must acquire the initial symbol synchronization within 5 msec, but the initial symbol synchronization may not be acquired within 5 msec since the synchronization can be acquired for each frame.